1. Field of the Invention
The present invention relates to a method and device for optical amplification.
2. Description of the Related Art
An optical communication system using an optical fiber transmission line is used to transmit a relatively large amount of information. A low-loss (e.g., 0.2 dB/km) optical fiber has already been produced and is being used as the optical fiber transmission line. In addition, an optical amplifier for compensating for loss in the optical fiber transmission line is used to allow long-haul transmission.
A conventional optical amplifier includes an optical amplifying medium pumped by pump light to provide a gain band. The optical amplifying medium and the pump light are selected so as to provide a gain band including the wavelength of signal light to be amplified. As a result, the signal light is amplified during propagation in the optical amplifying medium being pumped.
For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium, and a pump source for pumping the EDF. The pump source supplies pump light having a predetermined wavelength to the EDF. By presetting the wavelength of the pump light within a 0.98 xcexcm band or 1.48 xcexcm band, a gain band including a wavelength band of 1.55 xcexcm can be obtained. As a result, signal light having a wavelength band of 1.55 xcexcm is amplified.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are individually modulated by data. Each modulated carrier provides one channel of a WDM system for transmitting optical signals. These optical signals (i.e., the modulated carriers) are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light. The WDM signal light thus obtained is transmitted through an optical fiber transmission line to a receiving end. At the receiving end, the WDM signal light is separated into individual optical signals by an optical demultiplexer. Then, the original data can be detected according to these individual optical signals. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
In the case that an optical amplifier is inserted in a transmission line of an optical communication system adopting WDM, a transmission distance is limited by the noise characteristic of the optical amplifier and by the wavelength characteristic of gain which is represented by a gain tilt or gain deviation. In an EDFA, for example, a practical amplification band is present in a band of 1530 to 1610 nm, and a gain tilt is produced near this amplification band. It is known that this gain tilt varies with the total input power of signal light to the EDFA and the power of pump light.
To compensate the wavelength characteristic of gain of an optical amplifier, an optical filter is used as an equalizer. However, when the average gain of the optical amplifier is changed, the wavelength characteristic of gain largely changes. It is therefore necessary to perform control such that the gain becomes constant at a gain point where the optical filter is fabricated.
In an optical communication system, optical fiber transmission lines of various lengths are provided. Accordingly, the power of signal light to be input into an optical amplifier used as an optical repeater is not constant. It is therefore required to provide an optical amplifier which can support a wide input power dynamic range. Further, there is a case that a dispersion compensating fiber for compensating dispersion generated in an optical fiber transmission line is used. In this case, it is also necessary to change a level diagram in an optical repeater according to variations in loss in the dispersion compensating fiber.
In such gain control that the gain of an optical amplifier becomes constant as mentioned above, the power of output signal light changes with a change in the power of input signal light. However, the power of output signal light allowed to be supplied to an optical fiber transmission line downstream of the optical amplifier is limited by various nonlinear effects. Therefore, it is desirable that the output power from the optical amplifier is constant.
In these circumstances, there has been developed an optical amplifying device for maintaining the wavelength characteristic of gain constant and obtaining a wide input dynamic range. This optical amplifying device includes first and second optical amplifiers and a variable optical attenuator optically connected between the first and second optical amplifiers. Automatic gain control (AGC) is applied to each of the first and second optical amplifiers, thereby maintaining constant the wavelength characteristic of gain of each of the first and second optical amplifiers. Further, automatic output level control (ALC) is performed to the second optical amplifier by using the variable optical attenuator to thereby obtain a wide input dynamic range. That is, the output level of the second optical amplifier is maintained constant irrespective of the input level of the first optical amplifier, so that the input dynamic range of this optical amplifying device is widened.
However, this type of optical amplifying device is disadvantageous from the viewpoints of efficiency and noise characteristics, because undue loss is given by the variable optical attenuator. Particularly in the case that the amount of dispersion compensation is large, a dispersion compensating fiber having a length of ten and more kilometers is necessary. In this case, it is considered that a loss of about 20 dB may be incurred as the sum of the loss by the dispersion compensating fiber and the input dynamic range, causing a large degradation in efficiency of an amplifier repeater.
Further, with higher transmission speeds in recent years, the influences by nonlinear effects (self-phase modulation, cross-phase modulation, etc.) have become apparent as the cause of degradation in error rate. Particularly in the case that the transmission speed is 40 Gbit/s or more, it is necessary to minimize the influences by the nonlinear effects to signal light. In a repeater, the nonlinearity caused in a dispersion compensating fiber is large, so that it is necessary to reduce the power of signal light to be input into the dispersion compensating fiber, resulting in a degradation in noise figure in the repeater.
Further, there has been proposed a Raman amplifier having a Raman amplifying medium and a plurality of pump sources for pumping the Raman amplifying medium at different wavelengths, for the purpose of broadening the amplification band for signal light. As a method of controlling the wavelength characteristic of gain in the Raman amplifier, it is known that a monitor for monitoring the wavelength characteristic of optical transmission power of signal light passed through the Raman amplifying medium is used to feed back the result of this monitoring to each pump source, thereby controlling the wavelength characteristic of gain (Japanese Patent Laid-open No. 2001-15845).
In this method, however, it is necessary to correct for the power of interchannel crosstalk and spontaneous Raman scattering light in the monitored optical power, so there is a problem in control. Furthermore, it is necessary to use a spectrum analyzer for monitoring the signal light with high accuracy, inviting a disadvantage from the viewpoint of cost.
It is therefore an object of the present invention to provide a method and device for optical amplification which can maintain the wavelength characteristic of gain constant and can obtain a wide input dynamic range, thereby improving the amplification efficiency and noise characteristics. Other objects of the present invention will become apparent from the following description.
In accordance with an aspect of the present invention, there is provided a device comprising a first optical amplifying unit, a second optical amplifying unit, and a control unit. The first optical amplifying unit comprises a Raman amplifying medium and a first pump source for pumping the Raman amplifying medium. The second optical amplifying unit is optically connected to a rear stage of the first optical amplifying unit, and comprises an optical amplifying medium and a second pump source for pumping the optical amplifying medium. The control unit controls the gain of the first optical amplifying unit so that variations in output power of the first optical amplifying unit due to variations in input power of the first optical amplifying unit are canceled.
With this configuration, variations in output power of the first optical amplifying unit are canceled, so that the gain of the second optical amplifying unit can be maintained constant. Accordingly, the wavelength characteristic of gain of the whole device can be easily maintained constant, and a wide input dynamic range can be obtained. Moreover, the attenuation of a variable optical attenuator that may be sometimes provided between the first optical amplifying unit and the second optical amplifying unit can be reduced or nullified by the above control, thereby allowing the improvement in amplification efficiency and in noise characteristics.
In accordance with another aspect of the present invention, there is provided a method for optical amplification. This method is a method using the device according to the present invention. More specifically, the method according to the present invention comprises the steps of amplifying signal light by a first optical amplifying unit comprising a Raman amplifying medium and a first pump source for pumping the Raman amplifying medium; amplifying signal light output from the first optical amplifying unit by a second optical amplifying unit comprising an optical amplifying medium and a second pump source for pumping the optical amplifying medium; and controlling the gain of the first optical amplifying unit so that variations in output power of the first optical amplifying unit due to variations in input power of the first optical amplifying unit are canceled.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.